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Part One: Explanation of the Changes of the Enthalpy
This part of the analysis has described several conditions responsible for changes in the enthalpy. Here the different explanations will be elaborated based on the different reactions.
a) Definition of the term activation energy
The minimal amount of energy that is needed to begin the production of the various products by using the reactant molecules in the specific chemical reaction is called the activation energy. This barrier of energy should be overcome in the chemical reaction to get the response. The molecules of the reaction which are known as the reactant molecules required to gain the energy to enter the state of the transition phase. In this phase, the bonds of the chemical reaction can be disrupted to manufacture another product. So the reactant molecules of the reaction require the activation energy to get the energy and provide a response.
b) The difference between the exothermic and endothermic reactions
The differences between the exothermic reaction and the endothermic reaction give a precise explanation of this. In the reaction of exothermic, in the surrounding the energy can be emitted in the formation of the heat (Finkenstaedt et al. 2020). So the changes in enthalpy or ?H in the reaction of exothermic would be negative. This happened because of the lower energy of the product compared to the energy of the reactant molecules. On the other hand, in the case of endothermic reactions energy can be acquired from the surroundings. So for the endothermic reaction the changes in enthalpy or ?H give the result of positive. The reason for this result can be elaborated by the observation of higher energy in the product compared to the energy of the reactants.
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c) Discussion of the enthalpy for dissociation
An example of the endothermic reaction is the gaseous hydrogen iodine which dissociates into the hydrogen gas and the iodine gas. The gaseous hydrogen chloride is an example of the endothermic reaction which offers the enthalpy change in the positive results. This means that the absorption of the energy has happened during the chemical reaction. So the graph of the reaction displayed the amount of the energy of reacting agents, the state of transition, and the end products. At first, the substances that react are 2HI and have a definite amount of energy. So the activation energy is required to begin the reaction and enter into the transition state (Maia et al. 2021). When the reactants have reached the transition condition hydrogen iodide bonds have broken to form the new bond resulting in the final products including the hydrogen and iodine. As this is an endothermic reaction the energy of the products has been observed as higher than the reactant molecules. So the results indicate the positive enthalpy which can be absorbed at the time of the reaction.
d) Discussion about the exothermic reaction energy
The energy profile of the exothermic reaction shows the graph which denotes the reactant, activation energy, the state of the transition, the emission of the energy, and the final products of the reaction. In this reaction, it has been observed that the reactants get the energy of the activation and enter into the transition state to provide the products.
Figure 1: The energy profile of the exothermic reaction
(Source: provided)
In this type of reaction, the energy should be overcome for the reaction to initiate the process. When the transition state has come the chemical bonds have broken and the reaction gets stable (Zhao et al. 2020). So in the graph, it has been shown that the quantity of the energy has decreased and the excess amount of the energy emitted into the surroundings.
A) The standard conditions for changes in enthalpy
The standard conditions for changing the enthalpy are referred to as the temperature of 298 k or 25C and the pressure of 1 bar. So in these types of conditions, the elements should be considered in the reaction which can be depicted as the steadiest form of the elements at the specific pressure and temperature (Das, and Sen, 2020). These are the required standards that are considered as the changes in the condition of the enthalpy.
B) Definition of enthalpy changes in different notation
The three different changes of the enthalpy in different situations of the chemical reaction are described in this.
i) The enthalpy change of reaction
This is the state of the modification of enthalpy in which one molecular substance can react under the condition of the standard measurements, and this reaction can be formed with the uses of all reactants of the reaction and the end products.
ii) The enthalpy change of combustion
This situation can be depicted as if one molecular of substances goes through the complete process of combustion then the excess amount of the oxygen in the mentioned standard condition can be combusted.
iii) The enthalpy change of formation
This is the state of the changes of the enthalpy in which one molecule of substance can be formed (Barros et al. 2020). These are formed from the basic elements in the mentioned standard conditions for the modifications of the enthalpy.
C) The equation of the changes of enthalpy in different situations
i) The change of the enthalpy in the formation of pentan-1-ol
The changes of the enthalpy in the formation of pentan-1-ol can be described by the chemical reaction; the reaction is arranged in the following way. The changes in the enthalpy can be denoted as the ?Hf. The equation represents how the pentan-1-ol has formed.
C5H12 + 8O2 → 5CO2 (g) +6H2O (l)
ii) The change of enthalpy in the combustion of butan-1-ol
The change of enthalpy in the combustion of Butan-1-ol can be the standard condition of the combustion and this can be represented by the ?HComb (Brown, 2020).
C4H9OH (l) +6O2 (g) →4CO2 (g) +5H2O (l)
iii) The change of enthalpy in the formation of methane
The formation of the methane is represented by the following equation. The change of the enthalpy in this equation can be symbolized as the ?Hf.
C(s) +2H2 (g) →CH4 (g)
iv) The change of enthalpy in the combustion of octane
The combustion of octane by the following equation has been represented and the changes in enthalpy have been observed under the standard conditions (Stengler et al. 2020). Here the combustion of the octane can be represented as the ?Hcomb.
C8H18 (l) + 25O2 (g) →8CO2 (g) +9H2O (l)
a) The explanation of the mean bond enthalpy
Mean bond enthalpy, also known as average bond enthalpy or bond breaking enthalpy, is a measure of the energy of chemical bonds between two atoms of a molecule or compound. It represents the amount of energy essential to break a mole of a specific bond some in air at a given temperature and pressure (Bach, and Schlegel, 2021). The bond enthalpy is usually stated in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). It is a useful concept to discuss the relative constancy of molecules, understand the mechanisms of reaction, and calculate the enthalpy changes associated with chemical reactions. The higher the bond enthalpy value, the robust the bond, and the more energy is needed to break it. Conversely, lower bond enthalpy values specify weaker bonds that are less likely to break. Mean bond enthalpies are widely used in chemical thermodynamics, kinetics, and bond energy calculations.
b) Calculation of the changes of enthalpy in the combustion of pentan-1-ol
To calculate the enthalpy change of the burning of pentane-1-ol (CH3CH2CH2CH2CH2OH) by using the average bond enthalpy, must consider the bonds broken in the reactants and the bonds formed in the products around.
For pentan-1-ol, the bonds broken are represented in the following way;
9 C-H bonds (9 × 413 kJ/mol = 3,717 kJ/mol), 4 C-C bonds (4 × 347 kJ/mol = 1,388 kJ/mol), 1 C-O bond (1 × 365 kJ/mol = 365 kJ/mol) and 1 O-H bond (1 × 464 kJ/mol = 464 kJ/mol)
The bonds formed in the products (CO? and H?O) and their changes of the enthalpy has represented in this way; 5 C=O bonds (5 × 805 kJ/mol = 4,025 kJ/mol) and 6 O=O bonds (6 × 496 kJ/mol = 2,976 kJ/mol) So, the enthalpy change is calculated by subtracting the sum of the bond enthalpies of the bonds formed from the summation of the bond enthalpies of the bonds broken (Estes, and Powell, 2020). The change of the Enthalpy = (3,717 + 1,388 + 365 + 464) - (4,025 + 2,976) = -1,067 kJ/mol Therefore, the enthalpy change for the burning of pentan-1-ol is -1,067 kJ/mol, representative that the reaction is exothermic.
c) Calculation of the enthalpy formation of nitrogen trifluoride
To calculate the enthalpy of formation of nitrogen trifluoride (NF3) using the given mean bond enthalpies, take into description the bonds broken in the reactants and the bonds formed in the product. For the components, the calculation of the enthalpy can be represented as;
1 N≡N bond broken (1 × 945 kJ/mol = 945 kJ/mol) and 3 F-F bonds broken (3 × 159 kJ/mol = 477 kJ/mol)
The calculation of the enthalpy can be represented for the product has been also represented as;
3 N-F bonds formed (3 × 276 kJ/mol = 828 kJ/mol)
The enthalpy of formation is calculated by subtracting the total bond enthalpies of the broken bonds from the total formed bond enthalpies.
Enthalpy of formation = (945 + 477) - 828 = 594 kJ/mol Thus, the enthalpy of formation of nitrogen trifluoride (NF3) is 594 kJ/mol, indicating that the formation of NF3 from its products is an endothermic process, requiring energy addition (Laarhoven et al. 2021). It should be noted that these calculations take into explanation reactants and products, and may have slightly different values under diverse conditions or if more correct binding enthalpies are used.
a) Enthalpy of the reaction of bicarbonate of soda and hydrochloric acid
To determine the enthalpy of the reaction, when NaHCO3 reacts with a mixture of hydrochloric acid, calculate the heat released or absorbed during the reaction using the given data (Lazarou et al. 2021). A decease in temperature of 9.5°C specifies that the reaction is exothermic, releasing heat from the environments. Using the formula of heat released q = m × c × ΔT
It represented as
m = mass of the solution (assuming a density of 1 g/cm3, m = 25 g).
c = specific heat capacity of water (4.18 J/g·K)
ΔT = change in temperature (9.5°C)
Substituting the values : q = 25 g × 4.18 J/g·K × 9.5 K = 995.75 J
Since 3.8 g of NaHCO3 was used, calculate the enthalpy of the reaction per mole by dividing the het by the moles of NaHCO3 used.
Enthalpy of reaction = 995.75 J/ (3.8 g/84.01 g/mol) = -22.2 kJ/mol
Thus, when NaHCO3 reacts with a mixture of hydrochloric acid, the enthalpy of reaction is -22.2 kJ/mol, indicating an exothermic reaction.
b) Suggestion of the experiment
Experimental error: Inaccurate capacity of concentration, temperature, or other variables can cause discrepancies.
Assumptions: Mathematical assumptions, such as assuming that the weight of the solution is equal to that of water, can lead to error.
Heat loss: Some heat has been missing to the surroundings, resultant in a lower measured enthalpy change.
Incomplete process: If the process was incomplete, the measured temperature change would be smaller than predictable (Filimonov, and Filimonova, 2022).
Impurities: Impurities in the reactants or reaction mixture can affect the enthalpy change.
The experimental value of the enthalpy modification for the reaction of mixed NaHCO? with hydrochloric acid is lower than the standard enthalpy change found in the data book due to several possible reasons:
a) The significance of Hess’s law in the enthalpy
Hess’s law states that the total enthalpy change of a reaction is independent of the form, as long as the first and final states are the same (Sandler, and Woodcock, 2020). Based on the fact that enthalpy is a function of state, i.e. it depends only on the first and final states.
This rule is useful in defining enthalpy vales because it allows us to calculate the enthalpy change for a process where the enthalpy change is known or simply measured and cannot be measured directly by splitting it into steps. Hess’s law simplifies enthalpy calculations and provides a valuable tool for thermochemical calculations, especially when dealing with the direct study of reactions involving complex or unstable intermediates. This allows chemists to use available data and enthalpy values are determined indirectly by applying the principle of preservation of energy.
b) Calculation of changes in enthalpy by using Hess’s law
To calculate the enthalpy change of the formation of benzene (C?H?)) through the Hess cycle use the Hess law by integrating the known enthalpy changes of the related reactions (Feller et al. 2020). The data are presented gives standard enthalpies of formation for carbon (C), for hydrogen gas (H?), and for liquid benzene (C?H?).
The structure of benzene can be shown as:
6C(s) + 3H? (g) → C?H? (l)
Construction of the Hess cycle by merging the following steps:
6C(s) → 6C (g) (ΔH° = 6 × -394 kJ/mol = -2364 kJ/mol)
3H? (g) → 6H (g) (ΔH° = 3 × -286 kJ/mol = -858 kJ/mol)
6C (g) + 6H (g) → C?H? (g) (ΔH° = -3267 kJ/mol)
C?H? (g) → C?H? (l) (ΔH° = enthalpy of vaporization, assumed to be small)
Using Hayes' law, the sum of the enthalpy changes in these fractions gives the enthalpy of formation of liquid benzene:
ΔH°f (C7H7 (l)) = -2364 kJ/mol - 858 kgJ/mol - 3267 kgJ/mol = 49 kgJ/mol
Thus, the enthalpy change of formation of liquid benzene (C?H?) is approximately 49 kJ/mol.
c) Calculation of the reaction with the provided data
To calculate the enthalpy change of reaction (ΔH°rxn) for a given reaction through the Hess cycle, apply Hess's law by integrating the known enthalpy changes of formation (Jusniar et al. 2020).
The reaction given is: 2KHCO?(s) → K?CO?(s) + CO? (g) + H?O (l)
Construction of a Hess cycle by considering the following steps.
2KHCO?(s) → K?CO?(s) + CO? (g) + H?O (l) (ΔH°rxn = unknown)
K?CO?(s) → K?CO?(s) (ΔH° = 0)
CO? (g) + H?O (l) → CO? (g) + H?O (l) (ΔH° = 0)
2KHCO?(s) → K?CO?(s) + CO? (g) + H?O (l)
ΔH° = 2 × (-959 kJ/mol) - (-1146 kJ/mol) - (-394 kJ/mol) - (-286 kJ/mol)
= -92 kJ/mol
Using Hess’s law, the total enthalpy change in this step is equal to ΔH°rxn. Thus, for the reaction 2KHCO3(s) → K2CO3(s) + CO2 (g) + H2O (l), ΔH°rxn is -92 kJ/mol, representative that the reaction is exothermic.
Part Two Understanding of the factors affecting the chemical reaction
a) Definition of the rate of reaction
The term the rate of the reaction in the chemical reaction can be depicted as the speed at which the reactants can transform into new products after the specified time. This can be measured as the modification in the concentration of the products with the reactants. So this term can be defined as the enhancement of the concentration of the product or the reduction in the concentration of the reactants in the specified unit time.
b) Description of the collision theory
The description of the collision theory with the contributing factors to affect the speed of the reaction is elaborated in this portion. The contributing factors are the concentration of reactants, surface area, catalysts, and temperature.
The concentration of the reactants is directly proportionate to the rate of the chemical reaction according to the collision therapy. This can happen because the higher concentration can increase the rate of the frequency of the collision. If the collision between the reactants and the products can be increased then the rate of the reaction will also increase. The collision of the particles can be increased due to the enhancement of the temperature of the reaction which results in the higher proportionate of the collision energy which is equal to or greater than the activation energy. The surface area of the reactants can be enhanced due to the exposing of more particles involving the other reactants. The factor of catalyst is very significant in the understanding of the collision theory which can increase the speed of the reaction by offering the lower amount of activation energy.
c) Explanation of the collision between the particles
The curve of Maxwell- Boltzmann
The labeling of the axes of the curve
The reason for the constant curve in two temperatures
(Source: self-created)
Figure: The diagram of distribution curve at lower temperature
Diagram of the distribution curve at lower temperature
(Source: self-created)
Figure: Maxwell-Boltzmann distribution curve
The suggestion of the reaction of the colliding particles
The explanation of the working of the catalyst
Part Three Application of Le Chatelier’s principle for explanation of changes
a) The principle of Le Chatelier
b) Explanation of the dynamic equilibrium
c) The required condition for dynamic equilibrium
Explanation of the effect on the different position
i) In the condition of increased pressure
ii) In the condition of increased temperature
The conditions of the Haber process and explanation of the term
Part Four Understanding of the theory of the reactions of acid and base
a) According to the Bronsted Lowry
i) The definition of base
ii) The definition of the acid
b) The definition of alkali
c) Identification of acid and base in two reactions
d) Measurements of the acidity and alkalinity by pH scale
e) The differences between the strong and weak acids
The chemical equations for the different reactions and explanation of changes
i) The reaction between lithium hydroxide and nitric acid
ii) The reaction between barium carbonate and hydrochloric acid
iii) The reaction between potassium hydroxide and sulphuric acid
iv) The reaction between hydrochloric acid and barium
v) The reaction between ammonia and nitric acid
vi) The reaction between sodium hydroxide and butanoic acid
vii) The reaction between nitric acid and iron
Part Five Understanding of the Redox Processes
Explanation of the oxidation and reduction
a) the oxidation state of the different elements in compounds
b) The oxidation state of the elements in ions
a) the definition of Redox
b) The state of oxidation or reduction
Identification of oxidized or reduced elements in the reaction
Explanation of reason for being Redox Reaction
References
Journals
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